Color anodizing in an inorganic electrolyte

ABSTRACT

A process for the color anodizing of aluminum comprising subjecting the aluminum, as an anode to electrolysis in an aqueous electrolyte containing sulfuric acid and dichromate ions.

United States Patent [72] Inventor Bernard Ray Baker Spokane, Wash. [21]Appl. No. 805,825 [22] Filed Mar. 10, 1969 [45] Patented Dec. 7, 1971[73] Assignee Kaiser Aluminum & Chemical Corporation Oakland, Calif.

[54] COLOR ANODIZING IN AN INORGANIC ELECTROLYTE 16 Claims, No Drawings[52] US. Cl. 204/58 [5 1] Int. Cl C23b 9/02 [50] Field of Search 204/58,35

[5 6] References Cited UNITED STATES PATENTS I 3,180,806 4/l965Hollingsworth 204/29 3,023,l49 2/1962 Zeman 204/12 2,855,352 10/1958Ernst 204/58 Surface Treatment of Aluminum, by Wernick & Pinner, 3rdedition, 1964, gs. 356- 357, 385, 387

Surface Treatment of Aluminum by Wernick et al., 3rd ed., 1964, pages353- 355 Primary Examiner-John H. Mack Assistant Examiner-R. L. AndrewsAttorneys-James E. Toomey, Paul E. Calrow, Harold L.

Jenkins and Frank M. Hansen ABSTRACT: A process for the color anodizingof aluminum comprising subjecting the aluminum, as an anode toelectrolysis in an aqueous electrolyte containing sulfuric acid anddichromate ions.

COLOR ANODlZlNG IN AN INORGANIC ELECTROLYTE BACKGROUND OF THE INVENTIONHeretofore many methods have been employed to place a protective oxidecoating on aluminum surfaces. One of the more frequently used is theprocess of anodization wherein the aluminum surface as an anode issubjected to electrolysis in an electrolytic bath which is capable ofyielding oxygen on electrolysis. The aluminum surface protected iscommonly termed anodized. Most often the baths are acidic. As usedherein, the term aluminum includes high purity aluminum, variouscommercial grades of aluminum and aluminum base alloys.

To produce an abrasion resistant coating it has been necessary in thepast to anodize in sulfuric or chromic acid baths at low temperature,such as to 30 F., which entails considerable expense for refrigeration.

Under many circumstances, such as in architectural applications, whereaesthetic considerations are quite important, it is desirable to have acolored or hued surface on the aluminum. Several processes have beendeveloped to produce colored oxide coatings; however, only one has metwith real commercial success, that being the integral color-anodizingprocess basically described in US. Pat. No. Re 25,566, assigned to thepresent assignee. Methods such as dyeing a previously anodized aluminumsurface with organic dyes or introducing metallic salts or oxides into apreviously prepared porous oxide coating suffer from the inherentdisadvantage of requiring additional process coloring steps after theanodization, which increase the cost as well as inconvenience of theprocess. The dyeing has the additional disadvantage of producing a colorwhich fades when exposed to ultraviolet light and also a color which isdifficult to reproduce from batch to batch. The color anodizing processdescribed in US. Pat. No. Re 25,566, involves the anodization ofaluminum in an aqueous electrolyte containing sulfosalicylic acid andsulfuric acid. Other sulfonic acids such as sulfophthalic acid may beutilized in place of the sulfosalicylic acid. The color-anodizingprocess produces not only a wide range of colors which are light-fastbut also an abrasion resistant oxide coating without the prior artrequirement of'electrolysis at low temperatures. in general, the colorsso produced are uniform, reproducible and have a high aesthetic appealnecessary for architectural applications.

Although the integral color-anodizing process described above is aneffective method to produce a color-anodized coating on aluminumsurfaces, the chemicals used, i.e.' the aromatic sulfonic acids, arecomparatively expensive and amount to a large portion of the expensesinvolved in the color anodizing of aluminum articles.

The purpose of this invention is to provide an effective color-anodizingprocess for aluminum which utilizes low cost, readily availableinorganic chemicals in the electrolytic bath and which produces a widerange of architecturally desirable colored oxide coatings which arehighly abrasion and corrosion resistant.

Other purposes and advantages will become apparent from the ensuingdiscussion.

SUMMARY OF THE INSTANT INVENTION The present invention is directed to anovel electrolytic bath and to the process of color anodizing analuminum surface in the bath. More particularly, it is directed to thecolor anodizing aluminum in an aqueous electrolyte containing sulfuricacid and dichromate ions, preferably from an alkali metal dichromate.The process is most advantageously operated by subjecting the aluminumsurface as an anode to a substantially constant current density betweenl5 and 40 amps/ft until a peak voltage across the cell between 42 and 70reached, then maintaining this peak voltage at a substantially constantlevel until the desired color density and oxide thickness is obtained.

The electrolytic bath embodied in the present invention is an aqueoussolution preferably having a sulfuric acid concentration of from 0.06 to0.1M and having a dichromate concentration of from 0.10M to 0.40M.

DETAILED DESCRIPTION In accordance with the present invention, it hasbeen found that an aqueous solution of sulfuric acid and dichromate ionscan be advantageously utilized as an electrolyte for the integral coloranodizing of aluminum alloys. Furthermore, it has been discovered that afull range of light-fast, architecturally desirable colors can beobtained by anodizing aluminum and aluminum alloys in an aqueouselectrolyte containing sulfuric acid and dichromate ions.

The dichromate concentration can range as low as 0.] gram-moles/liter upto the saturation point. Any soluble dichromate is operable in thepresent process if the compound yields dichromate ions in the desiredconcentration ranges. Potassium and sodium dichromate are preferred,although ammonium dichromate, lithium dichromate and ferric dichromateare fully operable. A dichromate concentration between 0.l0 to 0.40gram-moles/liter is preferred. The sulfuric acid can vary between 0.05and 0.30 gram-moles/liter, but it is preferred to maintain the sulfuricacid concentration between 0.06 and 0.10 gram-moles/liter. Furthermore,it has been found that small additions of up to 5 grams/liter of nitricacid to the electrolyte accelerates the color formation, considerablyreducing the time necessary for anodizing.

In the process of this invention an aluminum article is immerged as theanode in an aqueous electrolyte containing the sulfuric acid anddichromate compound and subjected to anodization. The anodization cancomprise a plurality of electrical programs; however, a two stageanodizing process is easiest to control and probably the leastexpensive. The two stage process comprises first subjecting the aluminumarticle to a substantially constant current density between 10 and 70amp/ft until a peak voltage between 42 and 120 volts is reached, andsubsequently maintaining the voltage at substantially this peak leveluntil the desired color density and coating thickness is obtained. it isrecognized that other electrical programs, such as maintaining thecurrent density between l0 and 70 amp/ft at a substantially constantlevel until the desired color density and coating thickness is obtained,can also be employed to give substantially equivalent results.

in the two stage process it is preferred to maintain the current densityin the first stage at a substantially constant level between l5 and 40amp/ft and the voltage in the second stage at a substantially constantlevel between 42 and 70 volts. in the single stage process the currentdensity should be maintained between the same limits, Le. 15 and 40amp/ft. The bath temperature can range from 0 to 100 F. but it ispreferred to maintain the temperature between 60 to F.

In the present invention color generation within the oxide coating is afunction of two separate phenomena 'which are presently not completelyunderstood. A slight golden hue is developed at low voltages andprobably exists throughout the entire anodic oxide coating. This colorformation may be due to a slight extent to the occlusion of chromatewithin the anodic oxide structure. However, the level of chromateocclusion within the oxide coating, which normally runs between 0.6 and0.9 percent CrO, does not warrant a conclusion that the occlusion ofchromate is even a major cause of this color generation. The primarycolor generation in the present process results from the formation of adark band of anodic oxidenext to the substrate metal. The color densityof the final anodized product is dependent upon the thickness of thisdark band. The color depends upon the color-imparting constituentswithin the dark band. The dark band, however, does not form unless athreshold voltage greater than 42 volts is reached during anodizing.Thus, in all anodizing programs the voltage across the electrolytic cellmust at some time during the process exceed 42 volts, otherwise only theslight golden hue is developed.

Others such as Keeler in US. Pat. No. 1,574, 290 and Frasch in US. Pat.No. 2,338,924 have anodized metals in a mixture of sulfuric acid anddichromate ions, however, in these cases the color generation isprimarily or entirely due to the chromium oxide deposition on the metalsurface, which is in these cases magnesium. Speer in U.S. Pat. No.2,437,620, employed an aqueous electrolyte containing sulfuric acid anddichromate ions in the anodic oxidation of aluminum, but he failed torecognize that within the composition ranges of the present inventionintegral colored oxide coatings canbe obtained.

During the startup of the anodizing program a short period, usuallyabout 3 to 6 minutes, is required to attain the desired current density.This is commonly termed the run-in period. Care must be exercised duringthis period not to exceed the threshold voltage, because once thisvoltage is exceeded the dark band forms and this dark band ischaracterized by a relatively high resistance when compared with theoxide formed at the low voltages. if the dark band is formed during therun-in period a higher voltage is necessary during anodizing to overcomethe higher resistance of this layer, thus increasing the cost of theprocess.

It has been found that with the two stage anodizing process the anodicoxide coating becomes momentarily passive when the constant voltagestage commences. This passivation results in a severe depression of thecurrent density. However, after a short time the current densityrecovers a considerable portion of its level and thereafter slowly diesout to a much lower level. At a peak voltage of 65 volts, the recoverytime is in the range of several minutes, but above 70 volts the recoverytime is in the order of several hours.

It has been found that by adding small amounts of sodium sulfate to theelectrolyte the current density level after passivation can be increasedand the recovery time reduced. Up to 20 grams/liter of sodium sulfatehave been found effective. Although sodium sulfate is preferred becauseof its cost and availability, any sulfate concentration above thatproduced from the sulfuric acid will produce equivalent results. Thus,up to about 0. l gram-moles/liter of additional sulfate will beeffective.

As mentioned above and recognized in the art there are many interactingvariables which effect the color of the anodic oxide coatings formed inthe various electrolytes. in the present process lighter colors areobtained by increasing the dichromate ion concentration, increasing thesulfuric acid concentration, increasing the sodium sulfateconcentration, decreasing the current density, decreasing the voltageand increasing the temperature. Darker colors are obtained by theconverse. This is not to say, however, that all of the variables must bevaried in the same direction to obtain either the dark or the lightcolors. For example, in commercial practice the bath composition willremain constant and the colors will be obtained by varying the currentdensity, peak voltages and bath temperatures.

Although the dichromate concentration in the bath must be maintained ata level greater than 0.10 gram-moles/liter, preferably between 0.1 and0.4 gram-moles/liter, to produce an integrally colored anodic oxidecoating, the overall color appears to be most sensitive to changes inthe total sulfate and sulfuric acid concentrations. Therefore, incommercial processes the total sulfate and sulfuric acid concentrationswill have to be controlled within rather narrow limits to produceanodized products which match from batch to batch. During anodizing thealuminum concentration in the electrolyte increases due to the effect ofthe acidic electrolyte on the aluminum oxide coating. For efficientanodizing the aluminum concentration should not exceed 13 grams/liter.Moreover, it is recognized that a variable aluminum concentration below13 grams/liter will produce a variance in the color response to aparticularanodizing program, making it difficult to produce integrallycolored coatings which match from batch to batch. Thus it is preferredto maintain the aluminum concentration at a constant level below 13grams/liter within a maximum deviation of 10.5 grams/liter. This controlis conveniently accomplished by passing a portion of the electrolytethrough a cation exchange resin such as a sulfonated polystyrene. Thereaction of the acid electrolyte and the oxide coating reduces thehydrogen ion concentration of the bath and when the acid is added toreplenish the hydrogen ion concentration the sulfate concentrationincreases. This excess sulfate is herein considered as sulfate producedor provided by the sulfuric acid.

In the table, several examples are given to illustrate particularembodiments of this invention. To prepare the samples for anodizing theywere first cleaned in an inhibited alkaline solution, caustic etched ina solution containing 50 grams/liter sodium hydroxide, 1.5 grams/litersodium gluconate and 5 to 25 grams/liter aluminum for 20 minutes at 130F., rinsed, desmutted in a 35 percent nitric acid solution and finallyrinsed. Alloys one through three were then immersed in an electrolyticbath containing grams/liter potassium dichromate, 8 grams/liter sulfuricacid and 7 grams/liter sodium sulfate and subjected to low voltagesduring a run-in period of about 3 minutes to bring up the currentdensity to the desired level and then subjected to the two stageanodizing programs shown in the table. Alloy four was subjected to thesame process except the bath contained 60 grams/liter sodium dichromate(Na,Cr,0-,-2H=O), 7 grams/liter sulfuric acid and l l grams/liter sodiumsulfate.

As is shown in the table, a full range of architecturally desirablecolors are obtained. These integrally colored anodic oxide coatings arefully equivalent and some instances superior in both abrasion resistanceand corrosion resistance as those produced by the process employing anelectrolyte containing sulfuric acid and an aromatic sulfonic acid, suchas Deal et al. in U.S. Pat. No. Re 25,566. Several samples anodized inaccordance with the present invention were subjected to jet abrasiontests to determine the abrasion resistance of the oxide coatings. Theresults of these tests shown in the table indicate that the oxidecoatings have an average abrasion resistance of about 1.5 sec/spot/mil.These results are comparable with the abrasion resistance of coatingsproduced by the Deal et al. process which generally have an abrasionresistance between 1.0 and 1.7 sec/spot/mil.

The integrally colored oxide coatings produced by the present inventioncan be sealed according to conventional sealing practice.

As was discussed above, the addition of up to 5 grams/liter nitric acidto the electrolyte tends to accelerate the anodizing process. Forexample, Alloy four in the table was anodized at l5". C. at a constantcurrent density of 30 amps/sq. ft. for 8.5 minutes during which time thevoltage rose from 40 to 62 volts. The voltage was maintained at the62-volt level for 38.4 minutes to obtain a statuary bronze anodic oxidecoating. Another sample of this same alloy, prepared in the same manner,was anodized in an electrolytic bath of the same composition except thatL8 grams/liter of nitric acid was added to the electrolyte. The samplewas anodized at 30 amps/ft until a peak voltage of 55 volts was reachedand this voltage was then maintained at a substantially constant leveluntil the same statuary bronze anodic oxide color was obtained. With thenitricacid addition the statuary bronze was developed in 38 minutes.This is to be compared with the total period of 46.9 minutes withoutnitric acid a 20 percent reduction in anodizing time.

During the process of the present invention a hexavalent dichromate ionis reduced to the trivalent state, and this requires a periodicreplenishment of the dichromate ion to maintain the dichromate ionconcentration within the desired limits. Obviously, if the rate ofdichromate reduction is known, the dichromate compound may be addedcontinuously to the bath. The inventor has discovered that thedichromate reduction can be substantially eliminated by utilizing ametallic cathode which has been coated with a plastic material which isresistant to the corrosive environments of the electrolytic bath, saidcoating having a plurality of small (1 millimeter) holes to expose thebare metal substrate and allow current to flow. The perforation diametercan vary from 0.5 to 1.5 millimeters although 1 millimeter is preferred.The perforations may be of any shape, but preferably round and thediameter disclosed above is the maximum diameter. The perforationsshould be spaced between 4 and 10 millimeters throughout substantiallythe entire cathode surface. Suitable plastics include, but are notlimited to epoxy resins, polytetrafluoroethylene and polyvinyl chloride.Obviously any material which is resistant to the particular electrolyteem- 5. A process for forming integrally colored anodic oxide coatings onaluminum comprising subjecting said aluminum as the anode in an aqueouselectrolyte at a temperature between 60 and 90 F. containing in solutionat least 0.10 gramployed for anodizing and which has a high resistivity,for ex- 5 moles/liter of a compound selected from the group consistingample above 10 ohm-centimeter, can be used as a coating Of sodiumdichromate not more than 13 gram per liter alumaterial. Apparently, bydrilling a plurality of these small mlnum and potassium dichromate andfrom 0.05 to 0.30 holes the current density through these holes is quitehigh, and grammoles/lllef Sulfuric acid to a Substantially constant atthe high current densities, the diffusion of the hydrogen ion remdensity between 10 and 70 P until a P l to the metal substrate of thecathode occurs preferentially to 10 age between 42 and 100 311515reached and maintaining Said the dichromate difi i and th f hydrogen gasf peak voltage at a substantially constant level until the desired tionoccurs preferentially to the dichromate reduction. in one color andCoamlg thlckness are obtainedtest, an epoxy resin, Epibond No. 122 resinproduced by the T Precess of 5 wherel" aqueous electrolyte p plasticsCorporation was p|aced on an aluminum contains between 0.10 and 0.40gram-moles/liter of a comcathode base and l millimeter holes weredrilled through this 5 Pound Selected f h group conslstmg of Sodiumdichroplastic coating to the substrate metal at intervals of 5 mm. mateand pofasslum dlchrqmate and between 006 and 010 After 15 runs in apreferred electrolyte composition containgram'lmles/llter Sulfur":aclding 8 grams/liter sulfuric acid and 90 grams/liter potassium T P sof Claim 6 wherein the current density is di h the epoxy coated h was m]ki maintained atasubstantially constant level between 15 and 40satisfactorily and the coating was practically intact. As a comamPts/fizand P321k Voltage mamtained at a Substantially parison, the anodizedaluminum cathode, which had similar Constant level hem/661142 and 70Voltsholes drilled through the anodic oxide coating, ceased produc- Tprocess f l im 7 wherein the aqueous electrolyte ing hydrogen gas afterfour runs and began reducing the contains asulfate concentration inexcess of that produced by dichromate ions to the trivalent state.Moreover, the cathode the Sulfuric acid in amounts p l0 15 g m-m/literstructure discussed above may be utilized in the anodization of 9.The process of claim 7 wherein the aqueous electrolyte magnesium as wellas aluminum. contains nitric acid in amounts up to5 grams/liter. It isobvious that various modifications to the present 10. The process offorming an integrally colored anodic process can be made withoutdeparting from the spirit of the oxide coating on aluminum comprisingsubjecting said aluinvention or the scope of the appended claims. minumas the anode in the aqueous electrolyte at a tempera- EXAMPLES Bath Cur-Time temrent Peak to Total Total Oxide Abrasion peradenvoltpeak, time,eurthickresisttnre, sity, age, minminrent, ness, ance, sec./ a.s.i.volts utes utes a.h.s.f. Color mils spot/mil Alloy:

26 20 43. 2 27. 2 27. 2 0. 87 25 24 22.3 23 0.89 25 30 17.5 18 10 do0.90 20 24 60 18.1 23. 2 10 Light amber. 0.87 1- 20 30 50 11.4 20.4 10.do 0.88 20 30 60 11. 7 24. 4 10 Amber" 0.89 15 30 57 8.5 32. 4 10 d0 0.s0 15 30 57 7.9 47 12 Stat. bronze 1.08 16 36 56 5. 0 58 14 d0 l. 18 2024 50 17. 1 23. 0 10 Light ambeL 0.97 20 24 17.9 24.1 10 Amber 0. 90 220 30 56 11.0 22.6 10 do 1.03 20 30 59 11.3 82.8 12 Stat. bronze 1.12 1530 57 7. 2 49. 4 12 do 1.15 3 16 30 65 4.0 48 10 Brown. 0.93 4 15 30 628.6 46.9 10 Stet.bronze 1.00

I Alloy composition in weight percent: 0.10 Si, 0.54 Fe, 0.09 Cu, 0.75Mg, 0.06 Cr, 0.01 Ti, balance Al. 11 Alloy composition in weightpercent: 0.34 Si, 0.17 Fe, 0.25 Cu, 0.13 Mn, 0.51 Mg, 0.01 Ti, balanceAl.

9 Alloy composition in weight percent: 0.10 Si, 0. balance Al.

What is claimed is: H V

1. A process for forming an integrally colored anodic oxide coating onaluminum comprising anodizing said aluminum as the anode in an aqueouselectrolyte at a temperature between and 90 F., said electrolyteconsisting essentially of at least 0.1 gram-mole/liter of dichromateion, from 0.05 to 0.30 gram-mole/liter sulfuric acid, not more than 13grams/liter aluminum, and the balance water, the voltage during a periodof said anodizing exceeding at least 42 volts. 7 V

2. The process of claim 1 wherein the aqueous electrolyte contains insolution between 0.10 and 0.40 gram-moles/liter of a compound selectedfrom the group consisting of sodium dichromate and potassium dichromateand between 0.06 and 0.10 gram-moles/liter sulfuricacid. V g V 3. Theprocess of claim 2 wherein the aqueous electrolyte contains sulfate ionsin excess of that produced by the sulfuric acid in amounts up to 0.15gram-moles/liter. V v

4. The process of claim 2 wherein the electrolyte contains nitric acidin amounts up tofi grams/liter.

7 Fe, 0.06 Cu, 0.48 Mn, 4.00 Mg, 0.18 Cr, 0.05 Zn, 0.02 T1, 9 Alloycomposition in weight percent: 0.34 Si, 0.17 Fe 0.25 11, 0.13 Mn, 0.51Mg, 0.01 Ti, balance Al.

ture between 60 and F., said electrolyte containing in solution at least0.1 gram-mole/liter of a compound selected from the group consisting ofsodium dichromate and potassium dichromate, not more than 13 grams perliter aluminum, and from 0.05 to 0.30 gram-mole/liter sulfuric acid to asubstantially constant density between 10 and 70 amperes/ft, the voltageduring a period of said anodizing exceeding at least 42 volts, until thedesired color and coating thickness are obtained.

l l. The process of claim 10 wherein the aqueous electrolyte contains insolution between 0.10 and 0.40 gram-moles/liter of a compound selectedfrom the group consisting of sodium dichromate and potassium dichromate,and sulfuric acid between 0.06 and O. 10 gram-moles/liter.

12. The process of claim 10 wherein the current density is maintained ata substantially constant level between 15 and 40 amps/ft? 13. Theprocess of claim 12 wherein the aqueous electrolyte contains a sulfateconcentration in excess of that produced by the sulfuric acid in amountsup to 0. l5 gram-moles/liter.

balance water not more than 13 grams per liter aluminum.

16. An aqueous electrolyte suitable for the integral color anodizing ofaluminum surfaces consisting essentially of from 0.10 and 0.40gram-moles/liter of a compound selected from the group consisting ofsodium dichromate and potassium dichromate, sulfuric acid from 0.06 and0.10 grammoles/liter, nitric acid in amounts up to 5 grams/liter and thebalance water not more than 13 grams per liter aluminum.

2. The process of claim 1 wherein the aqueous electrolyte contains insolution between 0.10 and 0.40 gram-moles/liter of a compound selectedfrom the group consisting of sodium dichromate and potassium dichromateand between 0.06 and 0.10 gram-moles/liter sulfuric acid.
 3. The processof claim 2 wherein the aqueous electrolyte contains sulfate ions inexcess of that produced by the sulfuric acid in amounts up to 0.15gram-moles/liter.
 4. The process of claim 2 wherein the electrolytecontains nitric acid in amounts up to 5 grams/liter.
 5. A process forforming integrally colored anodic oxide coatings on aluminum comprisingsubjecting said aluminum as the anode in an aqueous electrolyte at atemperature between 60* and 90* F. containing in solution at least 0.10gram-moles/liter of a compound selected from the group consisting ofsodium dichromate not more than 13 gram per liter aluminum and potassiumdichromate and from 0.05 to 0.30 gram-moles/liter sulfuric acid to asubstantially constant current density between 10 and 70 amperes/ft2until a peak voltage between 42 and 100 volts is reached and maintainingsaid peak voltage at a substantially constant level until the desiredcolor and coating thickness are obtained.
 6. The process of claim 5wherein the aqueous electrolyte contains between 0.10 and 0.40gram-moles/liter of a compound selected from the group consisting ofsodium dichromate and potassium dichromate and between 0.06 and 0.10gram-moles/liter sulfuric acid.
 7. The process of claim 6 wherein thecurrent density is maintained at a substantially constant level between15 and 40 ampts/ft2 and the peak voltage is maintained at asubstantially constant level between 42 and 70 volts.
 8. The process ofclaim 7 wherein the aqueous electrolyte contains a sulfate concentrationin excess of that produced by the sulfuric acid in amounts up to 0.15gram-moles/liter.
 9. The process of claim 7 wherein the aqueouselectrolyte contains nitric acid in amounts up to 5 grams/liter.
 10. Theprocess of forming an integrally colored anodic oxide coating onaluminum comprising subjecting said aluminum as the anode in the aqueouselectrolyte at a temperature between 60* and 90* F., said electrolytecontaining in solution at least 0.1 gram-mole/liter of a compoundselected from the group consisting of sodium dichromate and potassiumdichromate, not more than 13 grams per liter aluminum, and from 0.05 to0.30 gram-mole/liter sulfuric acid to a substantially constant densitybetween 10 and 70 amperes/ft2, the voltage during a period of saidanodizing exceeding at least 42 volts, until the desired color andcoating thickness are obtained.
 11. The process of claim 10 wherein theaqueous electrolyte contains in solution between 0.10 and 0.40gram-moles/liter of a compound selected from the group consisting ofsodium dichromate and potassium dichromate, and sulfuric acid between0.06 and 0.10 gram-moles/liter.
 12. The process of claim 10 wherein thecurrent density is maintained at a substantially constant level between15 and 40 amps/ft2.
 13. The process of claim 12 wherein the aqueouselectrolyte contains a sulfate concentration in excess of that producedby the sulfuric acid in amounts up to 0.15 gram-moles/liter.
 14. Theprocess of claim 12 wherein the aqueous electrolyte contains nitric acidin amounts up to 5 grams/liter.
 15. An aqueous electrolyte suitable forthe integral color anodizing of aluminum surfaces consisting essentiallyof from 0.10 and 0.40 gram-moles/liter of a compound selected from thegroup consisting of sodium dichromate and potassium dichromate, between0.05 and 0.30 gram-moles/liter sulfuric acid, a sulfate concentration inexcess of that provided by sulfuric acid in amounts up to 0.15grams-moles/liter and the balance water not more than 13 grams per literaluminum.
 16. An aqueous electrolyte suitable for the integral coloranodizing of aluminum surfaces consisting essentially of from 0.10 and0.40 gram-moles/liter of a compound selected from the group consistingof sodium dichromate and potassium dichromate, sulfuric acid from 0.06and 0.10 gram-moles/liter, nitric acid in amounts up to 5 grams/literand the balance water not more than 13 grams per liter aluminum.